For dropdown settings, simply pick the default option again (the defaults are listed in this article per setting). For text settings, clearing the box and confirming stores an empty value — for most numeric settings an empty value behaves like 0/off, but it is not the same as 'never set'. For file settings, click the red X next to the file name; that removes the stored value entirely and the built-in default image is used again. When editing a specific kitchen, removing a file override makes that kitchen fall back to the Global value.